This invention relates to measurement of a layer of ice or other hydrogenous material and particularly to such measurement based on neutron moderation.
In the process of neutron moderation, fast neutrons collide with nuclei to loose energy and to be scattered. The most effective neutron moderator is a hydrogen nucleus which has almost the same mass as a neutron to result in a high rate of momentum transfer. Accordingly hydrogenous materials, such as ice which consists of H.sub.2 O molecules, are much more effective for reducing the energy of fast neutrons than are other materials which may be present. Correspondingly, the number of slow neutrons can represent information on the presence of proximate hydrogen nuclei and this information can be enhanced statistically by reducing the extraneous noise of fast neutrons. A measuring system for proximate hydrogen nuclei based on neutron moderation would include a source of fast neutrons and a selective detector of slow neutrons.
A convenient source of fast neutrons is a radium-beryllium mixture in which the radium is a source of alpha particles which react with beryllium to produce fast neutrons in the following nuclear reaction: EQU .sub.4 Be.sup.9 +.sub.2 He.sup.4 .fwdarw..sub.6 C.sup.12 +.sub.0 n.sup.1.
Conventional detectors of slow neutrons are based upon alpha emission from boron which occurs when the boron captures a slow neutron in the following nuclear reaction: EQU .sub.5 B.sup.10 +.sub.0 n.sup.1 .fwdarw..sub.3 Li.sup.7 +.sub.2 He.sup.4.
The alpha particle actuates a charged particle detector. The boron provides selective detection of slow neutrons since its neutron capture cross section is much larger for slow than for fast neutrons.
Alternative nuclear reactions for producing fast neutrons and detecting slow neutrons with related apparatus in a backscatter configuration may be found in U.S. Pat. No. 4,039,809 issued to S. M. Bailey. Although this gage shares many components with the present invention, its use for measuring the varying hydrogenous composition of substantially constant configuration of a bulk material differs from the measurement of a varying basis weight or thickness of a layer of material having a constant hydrogenous composition. Further, the prior Bailey gage is not structured for mounting to the skin of an airplane for measuring accumulation of a layer of ice.
A radiation gage for monitoring ice accumulation on aircraft with circuits to process signals for indication, warning, and de-icing is disclosed Dean et al. in U.S. Pat. No. 3,838,281. The gage, however, is of the transmission type where components are undesirably located on the outer surface of an airplane skin. Further, alternative types of radiation do not share with moderated neutron systems the selective discrimination against background radiation from the source and from scattering by aircraft structures. Such background radiation reduces accuracy.